Geochronology, Origin Of The Late Paleozoic Granitoids From The Altai Orogen In China And Their Geological Significance

The Altai orogen in China is a typical region of the Central Asian Orogenic Belt (CAOB), the world's largest Phanerozoic accretion orogenic belt and the most important site of crustal growth in the Phanerozoic. It is situated between the Sayan and Gorny Altai early Paleozoic arc belt in southern Siberia to the north and the Junggar block to the south. The Altai orogen in China contains very large volumes of granitoids and related rocks as CAOB. The granitoids occupy more than 40% of the present area of the orogen. New zircon U-Pb ages show that these granitoids were mostly emplaced during the early Paleozoic, and few in the late Paleozoic and Mesozoic.Based on previous studies, we made detailed field investigating, and study on zircon U-Pb chronology, mineralogy, petrology, REE and trace elements geochemistry, Nd, Sr and Pb isotopic geochemistry of the late Paleozoic granitoids and basic rocks from the Altai orogen. The following achievements and conclusions have been made.1,The Bulgen alkaline granite contains arfvedsonite and aegirine. The rocks are enriched in K ,Na, Al, REE and LILE (Rb , Th , Ba) with negative anomaly in Ba, Sr, Ti ,P and poor in Mg, Fe, Mn with high FeOT/MgO values. They have low Isr and high (positive) eNd(t) value(+ 1.3 + 6.4).All these characteristics suggest that the Bulgen alkaline granite could be formed by the mantle magma underplating, which induced the early subducted oceanic crust or arc remelting, and offered some mantle materials with strong crystal fractionation.2,Zircon U-Pb (SRHIMP) dating yielded a ²⁰⁶Pb/²³⁸U age of 358±4 Ma (MSWD=1.7) of alkaline granite (4001). Zircon U-Pb (La-ICPMS) dating yielded a ²⁰⁶Pb/²³⁸U age of 354±3 Ma(MSWD = 0.96)of alkaline granite (40042) and a ²⁰⁶Pb/²³⁸U age of 343±3 Ma (MSWD = 1.6) of biotite monzonitic granite (4053). They are same, so, the Bulgen alkaline granite formed in the early Carboniferous, which just follow the early Paleozoic calcalkali granitoids in time, and have some relationship in space with them. They constitute a calcalkali-alkali magmatic sequence, which show that magma characters changed from calcalkali to alkali during the late Devonian-early Carboniferous. It means that the tectonic setting changed in this period. Accounting for other geological events, all of them indicate that the Bulgen alkaline granite pluton formed in a post-orogenic setting, in which orogenic movement had been finished, and the Erqis Ocean had been closed at that time. After that, the Altai region changed to an extensional setting.3,Fifteen zircon U-Pb (TIMS,SHRIMP and La-ICPMS) ages show that, except for the early Carboniferous Bulgen alkaline granite, the late Paleozoic granitoids from the Altai orogen were mainly emplaced in the Permian. The Permian granitoids can be approximately divided into two stages (300 Ma 280 Ma and 280 Ma 250 Ma), they are mostly distributed in the southern part of the Altai orogen and few intruded into the central part of the orogen4,The Permian granitoids are composed of three groups: I-type granite, aluminous A-type granite-syenite and I-A intermediate type. I-type granite and aluminous A-type granite-syenite mainly formed during 300 Ma 280 Ma and distribute in the Erqis fault belt, some plutons have been deformed. At the same time, large volume mafic plutons (dykes) were emplaced. These mafic intrusions were composed of a"Bimodal"magma group with the granitoids. I-A intermediate type granitoids were mainly formed during 280 Ma 250 Ma, they were not only emplaced in the southern part of the orogen, but also intruded in the central part of the orogen, and undeformed. This Permian calcalkali-alkali serial magma formed under an uniform mantle underplating background, the mantle magma offer the heat and the material in different positions of different terrane , partly melted of mantle magma directly took part in the form of granitoids by magmatic differentiation and magmatic mixing.5,Most plutons of Permian granitoids in the Altai orogen are approximately circular, without deformation, showing passive emplacement. They usually cut the tectonic lineament, showing a typical post-tectonic feature. It shows that there is no strong regional deformation in this area at that time. So, the Permian granitoids in the Altai orogen were emplaced in a post-orogenic setting. Although there are some linear deformed granitoids in the southern part of the Altai orogen, they are only localized in the Erqis fault belt, and the deformation character is strike-slip. So, the dating of the deformed and undeformed pluton in the fault belt defines the Erqis fault belt deformation time (290 Ma 270 Ma).6,The late Paleozoic granitoids in the Altai orogen have high (positive) e Nd(t) values (e Nd(t) = + 1.3 + 7.3),and low TDM(2546 995 Ma). It indicates that mantle-derived material took part in the form of granitoid in the Phanerozoic. The Pb isotopic feature provides further support evidence for this. In addition, the e Nd(t) values of some granitoids are higher then the coeval ocean crust. It indicates that the Altai orogen occurrenced certain scale crustal growth and have both horizontal growth and vertical growth. The maximum amount of growth is 33,000 Km3,about 2.1% volume of the Altai area upper crust, and the minimum amount of growth is 25,000 Km3,about 1.5% volume of the Altai area upper crust.Based on the above study, some conclusions about the tectonic evolution modal of the Altai orogen in the Paleozoic can be drawed as follow:1) During the early Carboniferous, the Erqis Ocean wane to the close, the D-C1 stratum in the Altai orogen had been folded, the main Paleozoic orogenic epoch wane to the end. At 350 Ma, the Bulgen alkaline granite pluton intruded in the folded D-C1 stratum. It indicates the end of the orogenic event. The setting in this region changed to an extensional background, and turned into post-orogenic stage. The Erqis Ocean would have been closed,there would be some inland seas in some place.2) During the middle-late Carboniferous, this region might be turned into a relative quiet stage. There have not been found regional metamorphism and deformation and few granitoids emplaced at this stage have been discovered now.3) During the late Carboniferous-early early Permian, there are strong magmatic activities and large scale strike-slip tectonic movement, etc. The area entered in a full pull-apart setting. Actually, Most parts of the CAOB have strong magmatism during this period...